V. Radiation Effects on Initiation and Completion of Growth*t

The main activity of a plant seedling is growth. During early development, growth is mainly longi tudinal, so that a simple measurement of length re veals much about the growth processes. For this reason, plant seedlings have been widely used to study the effects of radiation on growth. We have been studying the responses to radia tion of Mung beans (Phaseolus aureus). We had previously investigated reactions of dry seeds and growing plants, studied dose-response relations, and attempted (unsuccessfully) to correlate dis turbances of growth with changes in mitotic ac tivity. The picture which is unfolding is far from simple: responses to radiation can be analyzed into a number of component reactions, which are a! tered differentially by changes in experimental conditions. This paper deals with the growth of the first two parts of the plant which grow upward, i.e., the hypocotyl and the first segment of the epicotyl. We have observed the linear growth of the whole plant afl(l of separate segments, after total or par tial irradiation. Two components of the growth process, growth initiation and growth compktion, are (listinguishable in normal plants (section I). Effects of resecting various parts show that growth initiation depends on food reserves stored in the cotyledons, whereas growth completion is locally autarkic, i.e., independent of food supplied from other parts of the plant (section II). Growth is temporarily inhibited by moderate doses of x-rays (section III). In addition, there is a permanent ef fect, which reduces the total amount of growth